If you leave them out, there is more area to wash whenever it gets dirty, and also any time you move it in, be sure to check the top of the slide for leaves and twigs that can damage the seal. Also, when open, it's more area to heat and cool. Your choice!

also, if you have your camp spot close to saltwater, You really have to be careful of Rust, I always brought mine in, but my folks lived in theres and after a few years the slid wouldnt come in at all.. The Coast may be Beautiful, But the weather will ruin anything thats not protected

I had my 5th wheel parked for a year at a resort. I would put in the slides once a month just to keep them operating. One thing I didn't do was cover the tires, went to move the camper and all 4 valve stems rotted. Not fun)